1. In chemistry, atoms or specific groupings of atoms that have gained or lost one or more electrons, as the “chloride ion” or “ammonium ion. ”

Ions are most familiar in aqueous solutions and in crystal structures, but they also exist in the gas phase at all altitudes in the atmosphere. They are most abundant, and have the greatest importance, in the ionosphere, between about 70 and 300 km in altitude. 2. In atmospheric electricity, any of several types of electrically charged submicroscopic particles normally found in the atmosphere. Atmospheric ions are of two principal types, small ions and large ions, although a class of intermediate ions has occasionally been reported. The ionization process that forms small ions depends upon two distinct agencies, cosmic rays and radioactive emanations. Each of these consists of very energetic particles that ionize neutral air molecules by knocking out one or more planetary electrons. The resulting free electron and positively charged molecule (or atom) very quickly attach themselves to one or, at most, a small number of neutral air molecules, thereby forming new small ions. In the presence of Aitken nuclei, some of the small ions will in turn attach themselves to these nuclei, thereby creating new large ions. The two main classes of ions differ widely in mobility. Only the highly mobile small ions contribute significantly to the electrical conductivity of the air under most conditions. The intermediate ions and large ions are important in certain space charge effects, but are too sluggish to contribute much to conductivity. The processes of formation of ions are offset by certain processes of destruction of the ions. See recombination, ion mobility.